1. Field of the Invention
This invention relates generally to wireless communication systems for devices implanted in the body, and more particularly to optical communication between an implanted device and a device external to the body.
2. Description of the Related Art
Implantable devices have become a standard method of treating various medical conditions, many of which relate to the heart. Examples of devices which are routinely implanted include pacemakers, defibrillators, and nerve stimulators. These devices and others which have not yet become routine (such as implanted personal identification chips) are being provided with large memories for storing vast amounts of data. In the case of medical devices, this data may include physiological data such as the electrogram (electrical waveform at the electrodes), instantaneous heart rate, blood pressure, volume pumped, body temperature, etc., and configuration data such as mode of operation, amplifier sensitivity, filter bandwidth, and error messages. Often the device stores data that has been collected over a period of hours or days. This data is periodically retrieved by a doctor to monitor the patient's condition and to monitor the device's status. In response, the doctor might re-program the device for a different mode of operation, sensitivity setting, etc.
A method is needed to retrieve this data rapidly. The retrieval needs to be rapid so as to minimize the inconvenience to the patient who will usually have to remain in the doctor's office for the data retrieval process. To download four megabytes of medical device data, for example, at 20 Kbit/s would take nearly a half-hour--an undesirably long time for both the patient and medical professional or technician.
One method for data retrieval is the use of electromagnetic coupling between a pair of coils. One coil is excited to induce a current in the other. Modulation of the excitation signal can be detected in the induced current, and so communication is achieved. The problem with this is bandwidth. The coils each have a self-inductance which acts to attenuate high frequency signals, so that the bandwidth of communications is limited.
Another method for data retrieval is to provide a direct electrical connection. A wire connected to the implanted device is passed directly through the skin and coupled to the external device. Inherent with this technique is increased discomfort and increased risk of infection.
Thus, another method is needed to transfer a large amount of data quickly from the implanted device to the external device with minimal discomfort.